(a) Conventional shell--Typically enclosures for modern desktop use in the office or home have been either:
nonstructural plastic skins over sheet-metal frames, as in usual manufacture of computers; or PA1 rigid shells enveloping the mechanism, as in usual manufacture of printers.
Both of these approaches produce structures that are expensive to make, due to relatively thick-walled parts that lead to subtle added costs for material, molding and storage. The first also produces structures that are heavy and so cost more to ship, and the second produces overall structures that are relatively large and so again cost more to store.
(b) Cantilevered mechanical modules--The second technique also has an even more severe disadvantage: the internal mechanisms in general are usually cantilevered within their shells. This construction invites mechanical deformation in event of shock loading, such as can occur during shipping--or even when the machine is moved within a home or office.
Dropping the machine a relatively short distance, or shoving the machine only moderately hard, creates relative acceleration between the chassis and the cantilevered mass. This relative acceleration can cause the mass to act on the cantilevering structure like a force operating through a lever--to exert much higher, damaging levels of force on the components in the region of attachment.
In other words, the deformation magnifies the shock load and so tends to aggravate damage. The relatively heaviness of the parts worsens this problem.
(c) Noncantilevered electronic assemblies--As to internal mounting of electronic boards and assemblies, in a sense the prior-art difficulties are opposite to those discussed above for mechanical modules (although part of the difference may be due to somewhat different usages of the word "cantilever"). Here the normal practice is to firmly screw down electronic modules at all corners, or along opposing edges--or both.
Such practice has three drawbacks. First, installation of screws to assemble electronics into a product is relatively time consuming. In a mass-production environment, each screw takes several seconds--translating into not only cost for labor but also significant aggregate added cost for workstations.
This consideration alone is sufficient motivation to seek mounting arrangements requiring fewer screws. It is a particularly important concern in view of rework requirements, because major electronic assemblies in image-related devices typically have a relatively high rejection rate.
Such assemblies usually carry a very large number of components, many or most of which are simply purchased at wholesale, off-the-shelf as finished products from independent manufacturers, and failure rates of all these components are multiplicative. Finished assemblies therefore constitute a major cause of product failure--even in the testing process that is associated with manufacture.
Accordingly it is common for a particular electronics module to require not only initial installation into a device but also subsequent removal from the device, and then reassembly of a replacement into the device, before the product leaves the factory. Of course this kind of occurrence multiplies the adverse effects of the screw mounting.
(d) Field service, and fastener time--As a second drawback, the same considerations continue into the after-market environment. The same kind of excess time consumption--in both disassembly and reassembly--are necessary when failure occurs in the field.
A very common way of dealing with functional failure, whether in a product purchaser's facility or in a service center, is to change out an electronics assembly that is responsible for the related function. Even in the field, failure of major electronics assemblies remains one of the highest sources of service events.
Of course such failures amount to an undesirable expense regardless of who pays for them, and the manufacturer wishes to cultivate a reputation for products that do not fail in the field--or at least that are not very expensive to fix if they do fail. Hence the manufacturer always has a strong motivation to avoid such failures.
Since it is the manufacturer, however, who most commonly also bears the expense directly, it is particularly natural for the manufacturer to seek manufacturing techniques that minimize the time consumption and therefore the cost of field service. Therefore it is doubly desirable to find manufacturing configurations that make electronic modules very fast and easy to put in and take out.
(e) Access--This third drawback is more peculiar to the aftermarket environment, in which a technician seldom enjoys the luxury of having the entire device initially disassembled on the workbench. Even in the production context, however, when final product testing indicates that an electronic board must be replaced, accessibility of the components for removal and replacement becomes very important.
As mentioned earlier, electronic printed-circuit assemblies are conventionally screwed down along opposite edges or at all corners. When an assembly is thus installed in a relatively crowded case, in common interior layouts other modules of the device obstruct access to at least some of the screws. Just such tight packing, however, is generally required for good economy, as well as space in the end-user's office.
In particular, if entry to the interior is provided only through a relatively narrow or shallow access port, ordinarily just some of the screws can be within easy reach for removal. Such configurations require extra time for removal of other modules, merely to reach mounting screws for replacement of the circuit board--and then to replace those other modules later.
A conceptually and functionally trivial task thus becomes an exceedingly onerous chore: the service technician faces the task of taking apart much of the apparatus, merely to change out a board. It is compounded by the risk of damage (especially hidden damage) to those other modules in the course of the removal and replacement.
Interestingly, this is so even with some devices whose cases come off completely for service--as for instance a typical personal computer. Replacing a motherboard, even when the technician can look straight down at almost all of it, may take twenty to forty minutes.
(f) Solutions to the mechanical problems--The previously mentioned patent document of Hong et al. introduces a radical approach to structure and enclosure design for desktop image-related devices. That document teaches integration of an external shell with an internal framework of several chassis that cooperate to distribute specifically anticipated shock loads in a predetermined way.
The resulting configurations make the most of both investment and weight, since the enclosure essentially functions as one of the structural elements--but without being in itself being heavy or rigid. The invention of the Hong et al. document also resolves several other knotty problems of the prior art, which are detailed in that document but may be summarized here:
vulnerability to shock that is transfered directly through an attachment of an exterior face to an internal chassis;
flexure or failure of shells not strong enough to withstand impact--so that impact passes directly through the shell to apparatus within; and
overly demanding requirements for time-consuming, costly alignment (and forcible distortion) to fit shells onto the chassis for installation of mounting screws.
(g) Broad metallic base--Since many image-related devices require high operating frequencies, e. g. for firing of multiple nozzles at high resolution, these devices are subject to stringent regulations on control of electromagnetic interference ("EMI"). Such constraints in turn call for very effective grounding, preferably at several different locations on an electronic assembly. A common approach to meeting these requirements is to screw the main circuit board to a broadly extended metallic chassis element, preferably at the bottom of the device.
To avoid the problems of access mentioned earlier, in a device with such a broad metallic pan or base, an enclosure is often made with bottom access--i. e. with the base itself removable (together with any shell below it) through the bottom of the product. The main circuit board is then taken out through the bottom, with the base.
This option is foreclosed, however, in an integrated structural/enclosure system such as that of Hong et al. previously discussed. Such a product does have a broad metallic base, but its function as an integral part of the structural system, cooperating in the interconnections of the various chassis and shell members, precludes its easy demounting and removal.
Hence it remains to be shown how an effective grounding strategy, as well as relatively quick and easy assembly and disassembly, can be provided for major printed-circuit assemblies while maintaining the benefits of an integrated enclosure-and-structural system. As will be understood, this problem interacts with the access problem discussed above:
If a solid, positive ground could be obtained without access to all the fasteners--at opposed sides--that hold the board in position and also ground it, then technicians would be able to install and remove printed-circuit assemblies much more quickly and easily. Both the grounding and the access would be greatly simplified and improved, particularly in an integrated enclosure-and-structural system.
An integrated system would then be comparable with, for instance, a conventional bottom-access system in ease and rapidity of service. In short, a fastenerless around for printed-circuit assemblies would significantly advance the field of image-related devices.
(h) Conclusion--Conventional approaches have continued to impede achievement of uniformly excellent electronic serviceability as well as mechanical integrity, in lightweight, economical cases and chassis for desktop printing machines. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.